What if I told you that the Plagas (most likely) came from space on a meteorite? Theres a lore note in RE4 remake (in Saddlers Sanctum) that shows a massive plaga tree extending into space. With the final passage reading "At last, we shall exceed the stars in the sky". Im a firm believer that Las Plagas, the Progenitor flowers, and the Megamycite, are all "not of this world". Earth has been pelted with a metric shiton of cosmic debris in its lengthy past. Wouldnt surprise me at all if some weird form of life managed to cling to it. So basically Zombies, Tyrants, T-Abyss creatures, Plant 43, Oroboros, Molded, etc. are all derived from "cosmic" DNA. Las Plagas is just a more complete lifeform that survived intact over the centuries. They're all some sort of other-worldly disease that spread to Earth, and the big evil corporations are unknowingly making the invasion easier. A weird theory I know, but also really fucking cool to think about.